Unidirectional progression through the cell cycle depends on the specific, rapid, and temporally controlled proteolysis of key regulators by the ubiquitin-proteasome system (UPS). E3 ubiquitin ligases confer substrate specificity to the UPS. Therefore, it is not surprising that alteration of the functions of these enzymes contribute to the development of a wide variety of diseases, including cancer. SCF (Skp1, Cul1, F-box protein) complexes (also known as Cul1-Ring-ligases or CRL1) represent a family of E3 ubiquitin ligases involved in crucial cellular pathways as gene transcription, protein synthesis, cell division, DNA-damage checkpoints, the circadian clock, and apoptosis. F-box proteins play a pivotal role in the SCF complex by functioning as receptors that directly bind to and recruit substrates. Although certain F-box proteins have been characterized, most of them have not yet been matched to their cognate substrates and, therefore, are defined orphan. FBXO11 is an orphan F-box protein involved in regulating cell fate determination, and mutations of the gene encoding this protein have been associated with the onset and progression of pathological conditions both in humans and in animal models. Using unbiased proteomic screens, we have identified Cdt2 as a novel, putative substrate of SCFFBXO11. Cdt2 belongs to the family of DCAF proteins that are the substrate receptors of multi-subunit E3 complexes known as CRL4 (Cul4-Ring- ligase). CRL4Cdt2 controls the degradation of key regulators of proliferation (Cdt1, p21, Set8, etc.) in both normal and cancer cells; however, the mechanisms controlling the degradation of Cdt2 itself have remained unknown. Under the present application, we will study the molecular mechanisms and pathways regulated by both FBXO11 and Cdt2. To this end we will address how, when (Aim1), and why (Aim2) the SCFFBXO11 control the degradation of Cdt2. Taking together, our studies will provide novel insights into the regulatory circuits that control cell proliferation and differentiation. Moreover, they may offer a platform for future studies to explore the role of FBXO11 and Cdt2 in various cellular proliferative disorders. The proposed research is an extension of the NIH grant R37-CA076584 (07/01/1-06/30/2016) and represents a collaborative effort with Dr. Mario Rossi, a Group Leader at the BioMedicine Institute of Buenos Aires-CONICET-Partner Institute of the Max Planck Society in Argentina, the low- to middle-income countries (LMIC) site associated with this application, where the largest part of the project will be carried out under the supervision of Dr. Rossi.